If I were to start selling a product on Gumroad , and I kept the price at $0 for a few days and got a few 5-star reviews by creating fake purchases , and then increased the price and start selling , would they detect the fraud and ban me?
You don't need to do that.
Offer the product for free ($0) for a limited time right here on this forum and ask members to do a review on Gumroad. Other creators do it all the time. Udemy is a good example. You're sure to get some reviews.
The feedback will be genuine and will be a good indicator of quality. If the reviews aren't what you hoped for, it means you have to improve your product.
If your product isn't that good, you may get a lot of refund requests, which isn't good for the merchant account. You'd do well to avoid such a situation.
Negative feedback is as good as positive feedback. It helps you to know what the buyer wants, not what YOU think they want.